Stinking lagoons of pig poo created by thousands of animals in giant pig farms can pollute rivers, poison groundwater and pump out clouds of methane and carbon dioxide. Using microorganisms to break down slurry makes sense for two reasons. The first is environmental protection, but the methane produced by anaerobic digestion can also be used to generate electricity. The problem was that no one has been certain which way of doing it makes the most electricity for the least greenhouse gas production.

Denmark is famous for lots of things, but one of them is pigs. A Danish team has analysed the various ways in which firms in that country treat pig manure and use it to generate electricity in systems such as anaerobic digesters or incinerators. In anaerobic digestion, bacteria break down waste material by warming it in an oxygen-free vessel, releasing methane which is used in gas turbines. Incinerators burn material to boil water and drive a steam turbine. The team found that for high-efficiency energy production, anaerobic digestion is the best answer. But if minimising greenhouse gas emissions takes priority, the best option was to separate the solid from the liquid waste, dry the solids and incinerate them.

Energy Production, Nutrient Recovery and Greenhouse gas Emission Potentials From Integrated pig Manure Management Systems. Waste Manag Res. Sep 1. 2009 doi: 10.1177/0734242×09338728
Improper management of pig manure has resulted in environmental problems such as surface water eutrophication, ground water pollution, and greenhouse gas emissions. This study develops and compares 14 alternative manure management scenarios aiming at energy and nutrient extraction. The scenarios based on combinations of thermal pretreatment, anaerobic digestion, anaerobic co-digestion, liquid/solid separation, drying, incineration, and thermal gasification were compared with respect to their energy, nutrient and greenhouse gas balances. Both sole pig manure and pig manure mixed with other types of waste materials were considered. Data for the analyses were obtained from existing waste treatment facilities, experimental plants, laboratory measurements and literature. The assessment reveals that incineration combined with liquid/solid separation and drying of the solids is a promising management option yielding a high potential energy utilization rate and greenhouse gas savings. If maximum electricity production is desired, anaerobic digestion is advantageous as the biogas can be converted to electricity at high efficiency in a gas engine while allowing production of heat for operation of the digestion process. In conclusion, this study shows that the choice of technology has a strong influence on energy, nutrient and greenhouse gas balances. Thus, to get the most reliable results, it is important to consider the most representative (and up-to-date) technology combined with data representing the area or region in question.